(1) Field of the Invention
The present invention relates to a method and apparatus for accurately measuring the weight of particulate material moving along a pathway at different flow rates, and in particular to a method and apparatus for precisely measuring the amount of particulate material moving along a pathway as the volume of material changes.
(2) Description of the Prior Art
Numerous solid particulate materials are packaged in various types of containers for sale to the ultimate consumer or to a downstream processor. A brief exemplary listing of such materials include plastic and metal components and parts; food items, such as cereals, corn meal, rice, spices, soybeans, and potato chips; and a variety of other materials, such as tobacco, plastic pellets, etc.
Generally, the material is fed as a continuous or discontinuous feed from a bulk supply source, such as a hopper, directly into the containers, intermediate collection hoppers, or directed to other processing stages. The apparatus normally includes a means for weighing the material at some point. Control of the feed rate is used not only for material packaged by weight, but also for items packaged by the number of items, since these latter items are frequently packaged on the basis of the weight of the number of items in a container, as opposed to counting the number of items packaged. Desirably, the weight of the material is determined while the material is moving, instead of stopping the process. Dynamic measurement devices are used for this purpose in which the force of the moving material against a surface is used to calculate the weight and other properties of the material.
Particularly suitable dynamic measurement devices are described in earlier U.S. Pat. No. 5,219,031, issued Jun. 15, 1993, U.S. Pat. No. 5,230,251, issued Jul. 27, 1993, and U.S. Pat. No. 6,679,125, issued Jan. 20, 2004, all to the present inventor, these patents being incorporated herein by reference in their entireties. Generally, the dynamic measurement device described in these patents is comprised of a curved weigh pan having inlet and outlet ends, and an inwardly curved surface extending between the pan ends. The pan is positioned to receive a continuous stream of solid particulate material tangentially at the inlet end, i.e., the material stream is directed substantially perpendicular to the radius of curvature of the pan at the upper end. As a result, the material flows around the curved surface of the pan without impacting the pan, eliminating errors due to the movement of the pan under impact forces.
The pan is mounted on the distal end of an elongated support arm, with the proximal end of the arm being attached to a suitable support at a distance from the pan. To permit pan displacement, the arm is either flexible or is pivotally attached at its proximal end. When material flows over the inwardly curved pan, an inward centripetal force, and a corresponding outward force, is exerted causing the pan to move outwardly. This outward movement, corresponding to the weight, or change in weight, of the material moving across the pan is measured by the displacement measurement instrument. A displacement measurement instrument, such as a transducer, is positioned to continuously measure the displacement of the pan. Thus, a dynamic measurement device is capable of measuring variations in the amount of material moving along a given pathway without interrupting the flow of material.
Since the outward force exerted against the pan is due entirely to the centripetal force, and is independent of any impact or frictional forces, measurement is highly accurate, and can be measured instantaneously and continually. For optimal measurement independent of non-centripetal force factors, the pivot point of the arm is preferably located so that a line extending from the arm to the pan approximately midway between the pan inlet and outlet ends is perpendicular to the radius of curvature of the pan. The dynamic measurement device is positioned to receive material falling tangentially along the inner surface of the pan at its inlet end.
Centripetal force is the inward force required to keep an object moving in a circular path. It can be shown that an object moving in a circular path has acceleration toward the center of the circle along a radius. This radial acceleration, called the centripetal acceleration, is such that, if an object has a linear or tangential velocity when moving in a circular path of radius R, the centripetal acceleration is V2/R. If the object undergoing the centripetal acceleration has a mass M, then by Newton's second law of motion, the centripetal force Fc is in the direction of acceleration. This is expressed by the formula: F=MV2/R, where F is the force exerted against the pan by the moving material, M is the mass of the material being measured, V is the velocity of the material as it crosses the pan, and R is the radius of curvature of the pan surface.
While the above apparatus works extremely well in most applications, it has been found that calibration may be required where there is a significant increase in the flow rate of the material being measured. Accordingly, there is a need for a means for automatically adjusting apparatus of the above type to take into consideration the flow rate of the material being conveyed.